1. Field of the Invention
This invention relates to a method of manufacturing stepped round rod-like blanks which are suitable for a non-magnetic stabilizer for oil drilling as one typical example of application, and more particularly to a method of uniformly imparting a high strength to such rod-like blanks by cold working.
2. Description of the Prior Art
Taking the stabilizer an oil drilling as for example, and in the case of the directional drilling method in which a number of holes are extended in a number of directions from a single rig, it is necessary to employ a drilling tool of a non-magnetic material over the length of about 500 m from the tip end of the tool in order to eliminate the influences of terrestrial magnetism in monitoring the position of the drilling tip end by means of a magnetic sensor, including a stabilizer of high strength and toughness as shown in FIGS. 1 and 2. The target values in the properties of the stabilizer are greater than about 70 kg/mm.sup.2 in 0.2% proof stress and less than 1.01 in specific magnetic permeability. In FIG. 2, the double-dot-and-dash line indicates the shape of a blank 2 for forming the stabilizer 1 with stem portions 3, an intermediate bulging portion 4 and shoulder portions 5.
The production of stepped round rods by precipitation hardening has usually met with difficulties in obtaining high strength and toughness in addition to unsuitably low resistance to stress corrosion cracking, unless hardening by cold working is resorted to. However, the cold working involves various technical restrictions and difficulties due to the particular shape of the material and the severe values in target properties. Set forth hereinbelow are the problems or difficulties involved in the various conventional cold working methods.